I don't have PSE 10, but what happens if you make an edit in PSE 10 and then just close it or quit. Do you get a dialog asking if you want to save changes? Has PSE 10 adopted the Lion automatic save behavior? Maybe it's automatically saved and you don't need to do anything but close the file.
For save as, as long as you don't change the name or location of the file, Aperture should see the edited version. You may get a warning about an existing file with the same name when doing this. Just accept it.
Keep in mind some of this is my speculation since I don't have PSE 10.
Here's the correct answer from Barbara Brundage in another thread. No one had mentioned this before:
If you have correctly set up PSE as your external editor in Aperture and are using it that way, then the problem lies in your save preferences in PSE. Go to Photoshop Elements>Preferences>Saving Files and set On First Save to save over the original file without showing you the Save As dialog box. That's the only way for image management systems like Aperture or iPhoto to know to create versions.